1. Field of the Invention
The present invention relates to a portable FM-CW radar device, more particularly to a device on an automobile which can measure a relative velocity and distance simultaneously with respect to a target object to avoid collision with other automobiles.
2. Description of the Related Art
A frequency-modulating continuous-wave (FM-CW) radar device modulates a high frequency signal, transmits the modulated high frequency signal, receives a reflection signal reflected by a target object, and modulates the frequency of the reflected signal with a signal generated by a local oscillator provided by branching a portion of the transmitted signal.
In recent years, the number of automobiles owned has increased. The number of traffic accidents, such as collision of automobiles, has increased year by year as a result. To reduce the number of traffic accidents, it is necessary to provide an automobile equipped with a safety device of a reasonable cost, for example, an automobile distance monitoring system which allows a driver to determine a potential for collision in advance.
A FM-CW radar system is conventionally used to measure a relative velocity and distance separately with respect to a target object. The above-mentioned radar system can measure the relative velocity and a distance with a simple signal processing circuit, and further a transmitter and receiver for it can be simply constituted.
The aforementioned radar system is used to enable general drivers to prevent collisions between automobiles, so the radar system must be made compact, transportable, and low in cost.
The principle of an FM-CW radar system is as follows. Frequency modulation is conducted on an oscillator by a triangular wave of several hundreds of Hz sent from a modulation signal generator. The frequency-modulated wave is transmitted from a transmitting antenna. A signal reflected from a target object is received by a receiving antenna, and then a frequency changer like a mixer conducts FM-detection on the receiving signal, wherein the frequency-modulation wave is used as a local oscillation signal.
At this time, a deviation of frequency (beat) is caused between the reflected wave sent from the object and the transmitting signal in accordance with the distance between the radar apparatus and the object and also in accordance with a Doppler shift caused by the relative velocity.
The frequency component f.sub.b of this beat is expressed by (Distance frequency f.sub.r dependent on distance).+-.(velocity frequency f.sub.d dependent on speed), so the distance and relative velocity can be measured by the deviation of the frequency.
In this case, the following equations are satisfied: f.sub.r =(4.DELTA..OMEGA./Tc)R and f.sub.d =(2f.sub.0 /c)v, where .DELTA..OMEGA. is the modulation width, T is the period of a modulation wave, c is the velocity of light, R is the distance from the radar apparatus to an obstacle (a target object), f.sub.0 is the central transmitting frequency, and v is the relative velocity with respect to the obstacle (the object).
In the case where this system is used as radar for automobile use, the measured distance is 100 m at most, and the relative velocity is approximately 100 km/h. In order to ensure a sufficient accuracy of measurement of distance, the maximum frequency deviation must be about 100 Mhz. In order to ensure a sufficient accuracy of measurement of relative velocity, a millimeter wave band must be used for the transmitting frequency band.
Incidentally, in the case of a conventional millimeter wave band FM-CW radar, frequency modulation of a very large amount of frequency deviation is conducted, so that an AM component having approximately the same frequency component as the modulation signal is overlapped with the frequency modulation wave due to the inclination of the characteristics of oscillating frequency versus electric power of a voltage control type of oscillator. This AM component is detected by a mixer. However, the frequency of a triangular wave to frequency-modulate, and the frequency of a reflected signal after frequency modulation of a reflecting wave has been conducted, are very close to each other. Accordingly, the receiving S/N ratio is deteriorated. Therefore, unless the transmitting output is increased, a necessary searching region can not be ensured. For this reason, expensive Impatt diodes and Gunn diodes must be used for the oscillator under present conditions.